“Vertical axis” crushers are used for instance for crushing coal or clinker. They are essentially made of a rotating track supporting rollers that are driven by the rotary motion of the track along the vertical axis. The material to be crushed is fed into a central supply channel and falls onto the track, where it is crushed and ground between the track and the rollers. The ground material is then retrieved from the periphery of the track.
Various types of roller shapes are possible, such as tapered or toric rollers. Document DE 44 00 797 A1 describes rollers of this shape where the wear parts are mechanically sealed by a mechanical fixation means.
Document JP 2001 129418 describes a mechanical crusher with wear parts provided at the outer surfaces of the rollers, that allow their easy replacement when grooves are showing signs of wear. These wear parts are inserted into a core in the form of an inverted T so that they perfectly adhere to the whole surface of the protrusions.
The techniques used in the German and Japanese documents do not use the technology of casting the protrusions. The techniques used in the German and Japanese documents do not use the casting technology of pouring a more ductile material around inserts allowing to fix the latter in the remaining mass of the roller.
Specific forms of vertical axis crushers are shown in FIGS. 1 and 2. This subject is described in more detail in patent EP 0 476 496 B1.
This patent describes among other things a particular configuration of the grinding rollers whose characteristic feature is that the wear surface is essentially made up of peripheral inserts in very hard materials with high wear resistance (such as cast iron with a high chromium level), mechanically sealed in a matrix of a ductile material.
In this version, the inserts are produced in advance with a protruding rib on at least one longitudinal side and are then placed side by side in a mould, leaving between them a space defined by the thickness of their ribs. The roller is produced in the form of a bimetallic part by a casting technique by pouring a more ductile material that ensures the mechanical connection of the inserts in the remaining mass of the roller formed by the ductile material.
The peripheral inserts thus being spaced by ribs relative to each other, the ductile material, during the manufacture of the roller by casting, may infiltrate between the inserts until the peripheral wear surface itself, thus ensuring that the inserts are well fixed in the parts. This arrangement leads to a succession of hard inserts that are essentially separated by a gap in a more ductile material (with the exception being the spacing ribs).
In order for the ductile metal to be able to continuously infiltrate between the inserts, the gap created between the inserts when they are positioned in the mould should progressively increase from the periphery towards the middle in such a way that the molten metal does not coagulate upon contact with the cold inserts, thereby preventing the complete filling of this gap with the ductile metal. However, due to the wear of the part in use, this arrangement changes, creating at the periphery a wider and wider ductile gap, resulting of the slope of the sides of this gap. This nevertheless causes the unfortunate effect of reducing the hard peripheral surface of the inserts and, as a result, of accelerating the wear of the part.
Moreover, in the case of grinding rollers, localised preferential wear is observed, generally on the outer edges of the roller. This wear affects the useful life of the rollers but also the quality of the ground product as well as the grinding yield since the support surface of the roller on the track is reduced and this is even more so since the track itself is subjected to wear in operation.
In the case where, according to a recent technical development, the inserts contain internal reinforcement of ceramic materials in order to reduce wear, the presence of an unprotected gap between the inserts means that in operation, a groove is created between the inserts, thereby detaching the ceramic reinforcement and chipping its edges. This phenomenon greatly reduces the efficiency of the ceramic reinforcement since it then produces very abrasive materials in addition to the fact that its potential wear resistance diminishes with its size.
In the above-mentioned patent, there is also a description (in relation to FIGS. 5 to 9) of an embodiment of a collar that compensates for the wear profile according to the generatrix.
To do this, the inserts do not extend in their longitudinal direction along the entire length of the generatrix so that they leave, on the outer edge of the rollers, a peripheral lug belonging to the support of ductile cast iron, making up the rest of the roller.
More rapid wear is thereby deliberately caused in this area of the roller in order to compensate for the fact that this area normally wears less rapidly. However, this method has the drawback that the wear on the lug of ductile cast iron exposes the end of the insert to chipping similar to that described above regarding the longitudinal edges of the insert, with the same detrimental consequences.